Electro-mechanical transducer

ABSTRACT

A moving coil transducer comprises a bobbin of magnetically soft ferro-magnetic material having a cylindrical portion wound with an actuating winding between end cheeks, each of the end cheeks being disposed in the gap between a pair of pole pieces comprising parallel plates having apertures shaped to admit the end cheeks and spaced apart at a distance approximately equal to the thickness of the end cheeks.

PIPE-5G2 DR 3&7555699 Cartwright Aug. 28, 1973 1 1 lnvemofl John Bryan Francis Cartwright 1,473,957 12/1965 France 310/15 Wolverhampton, England [73] Assignee: Sperry Rand Limited, London, T$$:;:fi?H:gg:

' England; a pan Interest Attorney-'- Arthur RaischJohn M. Kisselle et a1. [22] Filed: June 12, 1972 I [21] Appl. No.: 261,790

[57] ABSTRACT [30] Foreign Apphcauof r Dam A moving coil transducer comprises a bol abin of mag- June 15, 1971 Great Bntaln 28.023/71 ny Soft ferro magnetic material having a cylindri cal portion wound with an actuating winding between [3] $5.31 310/12, SIOHgSlO/g) end cheeks, each ofthe end cheeks being disposed in [58] Fat. 103 141/ 5 the gap between a pair ofpole pieces comprising paral 1 0 care 6 lel plates having apertures shaped to admit the end 0 cheeks and spaced apart at a distance approximately Reierences Cited equal to the thickness of the end cheeks. UNITED STATES PATENTS 8 Claims, 4 Drawing Figures 3,351,850 11/1967 Crawford et a] 310/15 X PATENIED M1 2 m 3.755 699 SHEE! 1 OF 2 FIG.2

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m "In um um ELECTRO-MECHANICAL TRANSDUCER The present invention relates to electro-mechanical transducers of the kind designed primarily to produce an output force in response to an electrical input.

Transducers of this kind are used, for example, in hydraulic systems where the operation of the system may be controlled by means of a pilot valve in which a hydraulic pressure difference is balanced by an externally applied force. One form of transducer that has been used for this purpose is a torque motor, which consists of an electric motor arranged so as to be capable of being operated in a continuously stalled condition. If a linear force is required instead of a torque, a crank is attached to the armature shaft. Such torque motors are frequently large and heavy in relation to the output force attainable, and have a large inertia, making them unsuitable for applications in which rapid response is required. An alternative arrangement comprises a moving coil positioned in the gap of a pot magnet, but this arrangement is limited in the power output available and may be expensive to construct because of the accuracy of machining required.

One form of the present invention provides a linear force motor capable of giving a substantial force output with reasonably low inertia, and is relatively inexpensive to manufacture.

According to the present invention, a moving coil transducer comprises a bobbin of magnetically soft ferro-magnetic material having a cylindrical portion wound with an actuating winding between end cheeks, each of the end cheeks being disposed in the gap between a pair of pole pieces comprising parallel plates having apertures shaped to admit the end cheeks and spaced apart at a distance approximately equal tolthe thickness of the end cheeks.

Preferably the bobbin is supported with its end cheeks in the pole piece gap by a pair of flexible diaphragms, the spring stiffness of the diaphragms providing the restoring force to maintain the end cheeks centralised in the pole piece gaps when no current is flowing through the actuating winding. In a convenient construction an axial rod of non-magnetic material passes centrally through the iron bobbin and is screwed at the end to take clamping nuts by means of which the bobbin is clamped to the diaphragms.

For the transducer to have a linear current/force characteristic, it is necessary to keep the pole pieces constantly magnetised with the innermost pole pieces being of one polarity, and the outermost of the opposite polarity.

In order to achieve this, preferably the innermost pair of pole pieces are connected by soft iron yokes, and the outermost pair are connected by soft iron yokes, and permanent magnets are arranged between the yokes of the inner pair and the yokes of the outer pair, thus maintaining the pole pieces with the required polarities. This arrangement has the advantage of providing a low reluctance return flux path through the pole pieces and yokes for the magnetisation of the bobbin.

In an alternative arrangement the innermost pole piece at each end may be coupled to the outermost pole piece of the other by means of a core carrying a magnetising winding. In operation these windings are energised by an electric current so as to produce the required polarities at the pole pieces.

In suchan arrangement, by supplying a constant magnetising current to the pole .piece windings, the

transducer can be caused to operate with a linear force/current characteristic, its sensitivity being adjustable by varying the current through the magnetising windings. Alternatively, if the actuating winding on the bobbin and the magnetising windings are supplied in series, the transducer will have a square-law characteristic. The motor may be supplied with alternating current in this case, provided that the frequency of the alternating current is high in relation to the natural frequency of oscillation of the moving part of the torque motor.

If the windings on the cores connecting the pole pieces and the actuating winding on the bobbin are supplied from different altemating' current sources of the same frequency, the arrangement functions as a phasesensitive detector giving no displacement from the-central position when the supplies are exactly out of phase, and a maximum displacement in one sense or the other when the supplies are in phase or exactly out of phase. If such an arrangement is supplied with direct current in both windings, it will function as an analogue multiplying device, the displacement or force being pr'oportionalto both the current in 'the magnetising winding of the cores and the current through the actuating winding on the bobbin.

The invention will be further described by way of example with reference to the drawings filed with the Provisional Specification, in which:

FIG. 1 is a longitudinal section through a transducer according to the present invention designed to function as a linear force motor,

FIG. 2 is a transverse section of the transducer of FIG. 1,

FIG. 3 is a diagrammatic view of the pole pieces, and their connecting yokes employed in the transducer of FIGS. 1 and 2, and

FIG. 4 is a diagrammatic view of the pole pieces and their magnetising cores in an alternative embodiment of the invention.

The Figures are diagrammatic only and portions have been omitted for clarity.

Referring first to FIGS. land 2, the body of the transducer consists of a housing, of which only the end plates, l and 2, appear in the Figures. These end plates support the armature 3 of the transducer, which is in the form of a soft iron bobbin, the bobbin being carried at each end by a flexible diaphragm 4, 5 mounted in an aperture in the end plates. Through the centre of the armature 3 is a rod 6 of non-magnetic steel which constituts the output shaft of the transducer, which is arranged to function as a linear force motor, and is provided with means, (not shown) for attachment to the spool of a hydraulic spool valve or other equipment with which the force motor is to be used. The bobbin 3 includes end cheeks 3a, 3b carries an actuating winding 7 connected to the input terminals ofthe motor (not shown). I

At each end of the transducer are a pair of soft iron plates 8, 9, and 10, 11, which form pole pieces and establish the magnetic gaps in which the end cheeks of the bobbin-shaped armature are free to move. The outermost two, 8, l l, are of the one polarity and the innermost two, 9, 10, are of the opposite polarity. The plates 8 and 9, and also theplates l0, and l I, are spaced apart by a distance which is very slightly less than the thickness of the end checks of the armature, and the apertures in the plates are slightly larger than the end cheeks, so that the armature can move freely without risk of touching them.

The outermost pair of pole pieces 8 and 11 are coupled together by a pair of soft iron yokes 14, 15 of triangular section, and the innermost pair by a pair of similar yokes 12, 13. These components are shown separately, for clarity, in FIG. 3.

To maintain the magnetisation at the magnetic gaps between the pole pieces, a pair of permanent magnets are employed acting on these yokes. Referring to FIG. 2, a channel-shaped magnet 16 running most of the length of the transducer is arranged to have its north pole abutting against the yoke 12 and its south pole against the yoke 15. A similar magnet 17 on the other side of the motor has its north pole abutting against the yoke 13 and its south pole abutting against the yoke 14. The magnets therefore keep the yokes and the pole piece plates magnetised with the polarities shown in FIG. 1.

In operation, if a current is applied to the actuating winding 7 in such a sense as to produce a north pole at the left hand end, and a south pole at the right hand end of the armature, in the view shown in FIG. 1, the left hand cheek of the armature is repelled by the pole piece 8 and attracted by the pole piece 9 whereas the right hand cheek is repelled by the' pole piece and attracted by the pole piece 11, thereby producing a force on the armature towards the right, which is transmitted by the output shaft 6 to the apparatus to which the force motor is associated.

In one example of an experimental force motor constructed substantially as described above, the overall length was 3 inches and the outside diameter was 2% inches, the armature was a bobbin of 50:50 Ni Fe with 1600 turns of 34 s.w.g. PVA self-bonding copper wire. The cheeks of the armature were 0.840 inch diameter and 0.200 inch disc thickness and the magnets 16, 17 were of Alcomax 3 (Registered Trade Mark) with their edges chamfered to fit together as shown in FIG. 2. The pole pieces 8, 9, 10, 11, and the yokes 12, 13, 14, 15, were of 50:50 Ni Fe.

In this motor, with a current of 300 milliamps, a force of l 1 pounds was exerted, and the force varied linearly with current at all smaller currents to an accuracy within the limits of measurement. The output force dropped to half value for a displacement of 0.02 inches either side of its normal position. The natural frequency of oscillation of the armature was approximately 75 cycles per second.

FIG. 4 shows an alternative transducer according to the invention. In this case there are a pair of pole pieces 18, 19, at one end, and a pair of pole pieces 20, 21 at the other end of the transducer, the outermost at one end, 18, being connected to the inermost at the other,

20, by magnetic cores, 22, 23, the other pair 19, 21, similarly being connected by cores 24, 25. These cores are each wound with a magnetising winding, one of which is indicated by the dotted outline at 26. The windings may be connected in series or parallel in such a sense as to produce north poles at the innermost pair of pole pieces, 19 and 20, and south poles at the outermost pair of pole pieces, 18 and 21, or vice versa. The iron components throughout are made of magnetically soft low hysteresis material so that their magnetisation, within limits set by magnetic saturation, is proportional to the current flowing through the coils.

If the magnetising windings and the actuating winding on the bobbin are supplied with direct current from independent sources, the force on the bobbin is proportional to each of the currents, so that the device acts as an analogue multiplier. If the coils are fed from a common source, by connecting them in series or in parallel, the device has a square-law characteristic, and if the coils are supplied with alternating current at a frequency high compared with the natural frequency of oscillation of the moving bobbin, then the device serves as a phase detector, or a square-law AC force motor, as the case may be.

What we claim is:

l. A moving coil transducer comprising a bobbin of magnetically soft ferro-magnetic material having a cylindrical portion wound with an actuating winding between end cheeks, each of the end cheeks being disposed in the gap between a pair of axially spaced magnetically linked pole pieces at each end comprising parallel plates having apertures shaped to admit the end cheeks and spaced apart at a distance approximately equal to the thickness of the end cheeks.

2. A moving coil transducer according to claim I in which the innermost pair of pole pieces are connected by soft iron yokes, and the outermost pair are connected by soft iron yokes, and permanent magnets are arranged between the yokes of the inner pair and the yokes of the outer pair to magnetically link the pole pieces and maintain the pole pieces with the required polarities.

3. A moving coil transducer comprising a bobbin of magnetically soft ferro-magnetic material having a cylindrical portion wound with an actuating winding between end cheeks, each of the end cheeks being disposed in the gap between a pair of pole pieces comprising parallel plates having apertures shaped to admit the end cheeks and spaced apart at a distance approximately equal to the thickness of the end cheeks, said bobbin being supported with its end cheeks in the pole piece gaps by a pair of flexible diaphragrns, the spring stiffness of the diaphragms providing a restoring force to maintain the end cheeks centralised in the gaps when no current is flowing through the actuating winding.

4. A moving coil transducer according to claim 3 having an axial rod of non-magnetic material passing centrally through the bobbin and screwed at its ends to take clamping nuts by means of which the bobbin is clamped to the diaphragms.

5. A moving coil transducer comprising a bobbin of magnetically soft ferro-magnetic material having a cylindrical portion wound with an actuating winding between end cheeks, each of the end cheeks being disposed in the gap between a pair of pole pieces comprising parallel plates having apertures shaped to admit the end cheeks and spaced apart at a distance approximately equal to the thickness of the end cheeks, the innermost pole piece at each end being coupled to the outermost pole piece at the other end by one or more soft magnetic cores carrying magnetising windings.

6. A moving coil transducer according to claim 5 in which the actuating winding is connected in series with the magnetising windings so that in operation the force acting on the bobbin is proportional to the square of the current passing through the transducer.

7. A moving coil transducer according to claim 5 in combination with two sources of direct current connected one to the actuating winding and the other to of the same frequency applied oneto the actuating winding, and the other to the magnetising windings, so that the force acting on the bobbin is a measure of the phase difference between the currents in the two coils. t i i i 

1. A moving coil transducer comprising a bobbin of magnetically soft ferro-magnetic material having a cylindrical portion wound with an actuating windiNg between end cheeks, each of the end cheeks being disposed in the gap between a pair of axially spaced magnetically linked pole pieces at each end comprising parallel plates having apertures shaped to admit the end cheeks and spaced apart at a distance approximately equal to the thickness of the end cheeks.
 2. A moving coil transducer according to claim 1 in which the innermost pair of pole pieces are connected by soft iron yokes, and the outermost pair are connected by soft iron yokes, and permanent magnets are arranged between the yokes of the inner pair and the yokes of the outer pair to magnetically link the pole pieces and maintain the pole pieces with the required polarities.
 3. A moving coil transducer comprising a bobbin of magnetically soft ferro-magnetic material having a cylindrical portion wound with an actuating winding between end cheeks, each of the end cheeks being disposed in the gap between a pair of pole pieces comprising parallel plates having apertures shaped to admit the end cheeks and spaced apart at a distance approximately equal to the thickness of the end cheeks, said bobbin being supported with its end cheeks in the pole piece gaps by a pair of flexible diaphragms, the spring stiffness of the diaphragms providing a restoring force to maintain the end cheeks centralised in the gaps when no current is flowing through the actuating winding.
 4. A moving coil transducer according to claim 3 having an axial rod of non-magnetic material passing centrally through the bobbin and screwed at its ends to take clamping nuts by means of which the bobbin is clamped to the diaphragms.
 5. A moving coil transducer comprising a bobbin of magnetically soft ferro-magnetic material having a cylindrical portion wound with an actuating winding between end cheeks, each of the end cheeks being disposed in the gap between a pair of pole pieces comprising parallel plates having apertures shaped to admit the end cheeks and spaced apart at a distance approximately equal to the thickness of the end cheeks, the innermost pole piece at each end being coupled to the outermost pole piece at the other end by one or more soft magnetic cores carrying magnetising windings.
 6. A moving coil transducer according to claim 5 in which the actuating winding is connected in series with the magnetising windings so that in operation the force acting on the bobbin is proportional to the square of the current passing through the transducer.
 7. A moving coil transducer according to claim 5 in combination with two sources of direct current connected one to the actuating winding and the other to the magnetising winding so that in operation the force on the bobbin is proportional to the product of the two currents.
 8. A moving coil transducer according to claim 5 in combination with a source of two alternating currents of the same frequency applied one to the actuating winding, and the other to the magnetising windings, so that the force acting on the bobbin is a measure of the phase difference between the currents in the two coils. 